Traditionally, a method that achieves focusing by moving a lens or lenses has been widely employed to implement a focusing system for varying the focal length or focus position of an optical system. However, this method requires the use of a lens driving mechanism, and therefore has the drawbacks that the focusing system becomes complex, and that relatively large power is required to drive the lens driving motor. This type of focusing system has the further drawback that impact resistance is generally low. In view of this, as a focusing system that does not require the use of a lens driving mechanism, a system has been proposed that achieves focusing by varying the refractive index of a liquid crystal lens (for example, refer to patent document 1).
The liquid crystal lens for implementing this prior art system is constructed by sandwiching a liquid crystal layer between two glass substrates having a patterned electrode and a common electrode. The patterned electrode comprises a center electrode and a plurality of ring electrodes, and each ring electrode is connected to the center electrode by a voltage drop resistor. A leader electrode connected to the center electrode is electrically isolated from the ring electrodes, and a variable resistor is connected to the leader electrode via a power amplifier. On the other hand, to a leader electrode connected to the ring electrode (the outer electrode), a variable resistor is connected via an amplifier. An AC power source is connected in parallel to these variable resistors. The variable resistors cause the AC voltage supplied from the AC power source to drop.
A voltage profile is formed across the liquid crystal layer by the voltage signals applied to the leader electrodes and the variable resistors that causes the voltage to drop. By adjusting the respective variable resistors, various voltage profiles can be generated across the liquid crystal layer.
A contour detection method is known for implementing an autofocus (automatic focusing) system for a video camera; this method extracts information corresponding to an image out-of-focus condition directly from the captured video image, and controls the lens to minimize the out-of-focus condition by using a hill climbing method (for example, refer to non-patent document 1). Various kinds of autofocusing apparatuses using this hill climbing control method have been proposed (for example, refer to patent documents 2, 3, 4, and 5).
To date, however, no reports have been published on the use of the hill climbing control method in achieving focusing by varying the refractive index of a liquid crystal lens. One possible reason for this is that, in the case of a liquid crystal lens, it takes considerable time to detect a focused position by the mounting climbing control. Assuming, for example, that there are 10 preset focus positions covering a range from near to far, a search is then made for a peak in the out-of-focus condition information in sequence from near to far or from far to near, and a maximum of 10 positions must be searched before the peak is found. In doing so, the time required to detect the focused position is compared between the method that moves the lens and the method that uses a liquid crystal lens.
In the method that moves the lens, first the lens is moved to a point corresponding to a certain position to acquire information corresponding to an out-of-focus condition at that position, and then the lens is moved to a point corresponding to the next position to acquire information corresponding to an out-of-focus condition; this action is repeatedly performed. In this case, the processing time per position is short, for example, about 67 milliseconds, so that the time required to detect the focused position is about 0.67 second (=67 milliseconds/position×10 positions) at the longest.
On the other hand, in the method that uses the liquid crystal lens, the refractive index of the liquid crystal lens is varied by varying the voltage (drive voltage) that is applied to the liquid crystal lens to drive the liquid crystal lens. Accordingly, first a drive voltage corresponding to a certain position is applied to the liquid crystal lens to acquire information corresponding to an out-of-focus condition at that position, and then a drive voltage corresponding to the next position is applied to the liquid crystal lens to acquire information corresponding to an out-of-focus condition; this action is repeatedly performed.
However, since there is generally a finite delay before the liquid crystal can respond to a change in applied drive voltage, the process for acquiring the out-of-focus condition information has to wait until the response of the liquid crystal settles after the drive voltage has been changed. Accordingly, the processing time per position is long, for example, about 500 milliseconds, so that it takes a maximum of about five seconds (500 milliseconds/position×10 positions) to detect the focused position.
Further, according to patent document 1, voltage is applied across the voltage drop resistors in the liquid crystal lens and, as a matter of course, there are cases where lower voltage is applied to one end than the other end. For example, if it is desired to operate the liquid crystal lens as a convex lens, lower voltage is applied to one leader electrode, and higher voltage to the other leader electrode.
In this case, depending on the liquid crystal material used for the liquid crystal layer, the time required until the transient response ends in the liquid crystal to which the lower voltage is applied becomes longer than the time required until the transient response ends in the liquid crystal to which the higher voltage is applied. In this way, when operating the liquid crystal lens as a convex lens, there will be no problem if the liquid crystal equally responds to applied voltages across the entire area of the liquid crystal layer. However, when there is a portion where the transient response is slower, the time required before the liquid crystal lens can function as a convex lens is determined by the response time of the liquid crystal to which lower voltage is applied.
In particular, when it is required to maximize the power of the liquid crystal lens, the voltage difference between the center electrode and the outer electrode must be maximized. This requires that voltage be as low as possible and yet exert an effective force on the liquid crystal molecules in the liquid crystal layer be applied to the side to which the lower voltage is applied, and this has led to the problem that it takes a long time to achieve a lens having a desired refractive index profile in that portion (i.e., until the transient response ends).
Furthermore, to increase the power of the liquid crystal lens as much as possible, the birefringence of the liquid crystal material must be made larger, or the thickness of the liquid crystal layer must be increased. However, a liquid crystal lens constructed in this manner has had the same problem as described above, that is, the response of the liquid crystal is slow, and it takes a long time to achieve a lens having a desired refractive index profile.
Further, the response speed of the liquid crystal to an applied voltage varies depending on the temperature. As a result, there has been the problem that, depending on the operating environment, it takes a long time for the liquid crystal lens to have a desired refractive index profile.
[Patent Document 1] Japanese Patent No. 3047082
[Patent Document 2] Japanese Utility Patent Publication No. H02-44248
[Patent Document 3] Japanese Patent No. 2742741
[Patent Document 4] Japanese Examined Patent Publication No. H01-15188
[Patent Document 5] Japanese Examined Patent Publication No. H02-11068
[Non-patent Document 1] Kentaro Hanma and four others, “Contour Detection Autofocus System,” Institute of Television Engineers of Japan, Technical Report, Nov. 29, 1982, pp. 7-12.